High Definition Television (HDTV) continues to make progress in its attempts to replace conventional television. Paving the way for this progress are various companies and associations working on standards to provide for a global market for HDTV.
One such group of companies is known as the "Digital HDTV Grand Alliance" including members such as AT&T, David Sarnoff Research Center, Massachusetts Institute of Technology and others. A comprehensive overview of the strides made by this group are presented in an article by Robert Hopkins entitled "Digital Terrestrial HDTV for North America: The Grand Alliance HDTV System" published in the IEEE Transactions on Consumer Electronics (Summer 1994). This article is herein incorporated by reference for all of its teachings regarding the background and basics of HDTV systems including the use of Program and Transport Packet Streams.
In addition to the Grand Alliance, much effort has been expended by the Moving Pictures Expert Group (MPEG), a committee within the International Standards Organization (ISO), in attempts to establish various standards for the storage and transmission of HDTV data (e.g., MPEG-2 standards--formats for Transport Packet Streams). Accepted standards are periodically published such as the Video Section of Information Technology--Generic Coding of Moving Pictures and Associated Audio ISO/IEC 13818-2 (November 1994) (hereinafter "Video Section") and the Systems Section of Information Technology--Generic Coding of Moving Pictures and Associated Audio ISO/IEC 13818-1 (November 1994) (hereinafter "Systems Section") both of which are herein incorporated by reference for their teachings regarding established standards and formats including "stuffing" techniques.
The syntax for the MPEG-2 standard defines several layers of data records which are used to convey both audio and video data. For the sake of simplicity, the decoding of the audio data is not described herein. Encoded data which describes a particular video sequence is represented in several nested layers, the Sequence layer, the Group of Pictures layer, the Picture layer, the Slice layer and the Macroblock layer. To aid in transmitting this information, a digital data stream representing multiple video sequences is divided into several smaller units and each of these units is encapsulated into a respective packetized elementary stream (PES) packet. For transmission, each PES packet is divided, in turn, among a plurality of fixed-length Transport Packets. Each Transport Packet contains data relating to only one PES packet. The Transport Packet also includes a header which holds control information, sometimes including an adaptation field, to be used in decoding the transport packet.
When an MPEG-2 encoded image is received, a transport decoder decodes the Transport Packets to reassemble the PES packets. The PES packets, in turn, are decoded to reassemble the MPEG-2 bit-stream which represents the image in the layered records, as described above. A given transport data stream may simultaneously convey multiple image sequences, for example as interleaved transport packets. This flexibility also allows the transmitter to switch among formats providing material in 4 by 3 aspect ratio according to one standard and widescreen (16 by 9) material according to another standard.
Turning to a system implementation for delivering HDTV using MPEG-2 standards to the consumer, in general, as illustrated in high-level block diagram of FIG. 1, on the transmission side, video and audio signals are input to respective encoders 110 and 112, buffered in buffers 114 and 116, delivered to the system coder/multiplexer 118, and stored in storage unit 120 or transmitted by transmitter unit 120. On the receiving side, the signals are received by a system decoder/demultiplexer 122, again buffered in buffers 124 and 126, then decoded by decoders 128 and 130 and output as the original video and audio signals.
An important aspect of the illustration of FIG. 1 is that, although the intermediate stage buffering of the signals includes a variable delay, the overall delay from input to output of the signals is required to be substantially constant. This is accomplished by monitored flow control and buffers.
As indicated in FIG. 1, the delay from the input to the encoder to the output or presentation from the decoder is constant in this model, while the delay through each of the encoder and decoder buffers is variable. Not only is the delay through each of these buffers variable within the path of one elementary stream, the individual buffer delays in the video and audio paths differ as well. Therefore, the relative location of coded bits representing audio or video in the combined stream does not indicate synchronization information. The relative location of coded audio and video is constrained only by a System Target Decoder (STD) model such that the decoder buffers must behave properly; therefore, coded audio and video that represent sound and pictures that are to be presented simultaneously may be separated in time within the coded bit system by as much as one second, which is the maximum decoder buffer delay that is allowed in the STD model. Similar to the STD model is a Video Buffering Verifier (VBV) which, as stated in the Video Section:
Constant rate coded video bitstreams shall meet constraints imposed through a Video Buffering Verifier (VBV) defined in this clause . . . PA1 The VBV is a hypothetical decoder, which is conceptually connected to the output of an encoder . . . Coded data is removed from the buffer as defined below. It is required that a bitstream that conforms to this specification shall not cause the VBV to overflow. When low.sub.-- delay equals 0, the bitstream shall not cause the VBV buffer to underflow . . .
A high-level illustration of an exemplary STD model operating in conjuntion with an encoder is shown in FIG. 2.
The requirement that the VBV buffer or STD model decoders not underflow is very important as product quality is at stake. In order to maintain constant bitrate video, "stuffing" is implemented within various aspects of the system. "Stuffing" is the act of filling the data stream with "don't care" information simply to maintain the required bit-rate.
For Transport Stream packets carrying PES packets, stuffing is used when there is insufficient PES packet data to fill the Transport Stream packet payload bytes to a level that would support the transmitted data rate.
Stuffing, for example, can be accomplished by defining an adaptation field longer than the sum of the lengths of the data elements in it, so that the payload bytes remaining after the adaptation field exactly accommodates the available PES packet data. The extra space in the adaptation field and/or payload can be filled with stuffing bytes.
FIG. 3 shows the format and field locations for a Transport Packet Stream where each Transport Packet includes a Header and a Payload. The header of a Transport Packet includes fields for indicating the existence and controlling the length and content of an adaptation field. Within that adaptation field, another field is designated as "stuffing bytes". Stuffing bytes are similarly used in the payload of the Transport Packets.
As mentioned, however, using stuffing bytes, which typically comprise all logical one values (i.e., "11111111") in the Transport Header and all logical zero values (i.e., "00000000") in the Transport Payload, is a waste of system resources (e.g., transmission bandwidth). Accordingly, it would be desirable to make more efficient use of the system resources which, to date, have been limited to "stuffing".